CONVENTIONAL TRICKLING FILTER EFFLUENT QUALITY: PREDICTABILITY AND VARIABILITY

A survey conducted of eight Conventional Trickling Filter (CTF) plants showed that seven of them exceeded EPA secondary treatment regulatory requirements in at least one month of the year surveyed for each plant. These regulations limit monthly average concentrations of both biochemical oxygen demand (BOD5) and suspended solids (SS) to 30 mg/L. For cases where the total organic loading (TOL) was limited to less than 0.58 Kg BOD5/m3·d (36 lb BOD5/1000 cu ft·day), average monthly effluent SS values were over the regulatory limit 12 percent of the time. For cases where the TOL was greater than 0.58 Kg BOD5/m3·d, the average monthly effluent SS regulatory limit was exceeded 21 percent of the time. Because of seeding of nitrifiers into the BOD test, CTF effluent BOD5 values will frequently exceed the EPA secondary treatment regulatory limit. Of those seven plants reporting BOD5 rather than carbonaceous BOD5 (CBOD5), fully 63 percent of the monthly data points exceeded 30 mg/L.The problems with BOD5 values exceeding regulatory limits can largely be attributed to nitrification occurring in the BOD bottle, due to seeding of nitrifiers from the CTF. The best solution to address this is to seek a permit based on CBOD5.The CTF process seems capable of a high conversion of soluble BOD5 to biomass but accomplishes bioflocculation only inefficiently. As a result the principal focus in CTF process design should be on predicting effluent SS after secondary clarification, rather than effluent carbonaceous BOD5. If the TOL is set in the proper range, the effluent carbonaceous BOD5 is primarily determined by the effluent SS.There are several means to reduce CTF effluent solids, some better proven than others. These include reducing the BOD loading on the CTF process by the use of Chemically Enhanced Primary Treatment (CEPT), use of flocculator clarifiers to enhance the flocculation of trickling filter underflow solids, chemical addition to flocculator clarifiers, and conversion to the TF/SC process.A case example is presented comparing the CTF process to the TF/SC process for the construction of a new 60 mgd TF facility at OCSD's Plant No. 2. For the case of this large plant, the TF/SC process proved less costly, consumed less space and was more reliable in terms of effluent quality, and therefore was selected for implementation.